This invention generally relates to three-dimensional ultrasound anatomical imaging and, more particularly, to three-dimensional diagnostic imaging of a body by detecting ultrasonic echoes reflected from a scanned volume in the body.
Conventional ultrasound scanners create two-dimensional B-mode images of tissue in which brightness of a pixel is based on intensity of the echo return. Alternatively, in a color flow imaging mode, the movement of fluid (e.g., blood) or tissue can be imaged. Measurement of blood flow in the heart and vessels using the Doppler effect is well known. The phase shift of backscattered ultrasound waves may be used to measure velocity of the backscatterers from tissue or blood. The Doppler shift may be displayed using different colors to represent speed and direction of flow. In power Doppler imaging, the power contained in the returned Doppler signal is displayed. Although the following disclosure refers predominantly to B-mode imaging for the sake of brevity, the present invention applies to any mode of ultrasound imaging.
Two-dimensional ultrasound images are often difficult to interpret due to inability of the observer to visualize the two-dimensional representation of the anatomy being scanned. In addition, it may not be possible to acquire the precise view needed to make a diagnosis due to probe geometry or poor access to the area of interest. However, if the ultrasound probe is swept over an area of interest and two-dimensional images are accumulated to form a three-dimensional data volume, the anatomy becomes much easier to visualize for both the trained and untrained observer. Moreover, views which cannot be acquired due to probe geometry or poor access to the area of interest can be reconstructed from the three-dimensional data volume by constructing slices through the volume at the angle that is difficult to obtain.
In order to generate three-dimensional images, the imaging system computer can transform a source data volume retrieved from memory into an imaging plane data set. The successive transformations may involve a variety of projection techniques such as maximum, minimum, composite, surface, or averaged projections made at angular increments, e.g., at 10xc2x0 intervals, within a range of angles, e.g., +90xc2x0 to xe2x88x9290xc2x0. Each pixel in the projected image includes the transformed data derived by projection onto a given image plane.
In free-hand three-dimensional ultrasound scans, a transducer array (1D to 1.5D) is translated in the elevation direction to acquire a set of image planes through the anatomy of interest. These images can be stored in memory and later retrieved by the system computer for three-dimensional reconstruction. If the spacings between image frames are known, then the three-dimensional volume can be reconstructed with the correct aspect ratio between the out-of-plane and scan plane dimensions. If, however, the estimates of the inter-slice spacing are poor, significant geometric distortion of the three-dimensional object can result.
A conventional ultrasound imaging system collects B-mode, color flow mode and power Doppler mode data in a cine memory on a continuous basis. As the probe is swept over an area of the anatomy, using either a free-hand scanning technique or a mechanical probe mover, a three-dimensional volume is stored in the cine memory. The distance that the probe was translated may be determined by any one of a number of techniques. The user can provide an estimate of the distance swept. If the probe is moved at a constant rate by a probe mover, the distance can be easily determined. Alternatively, a position sensor can be attached to the probe to determine the position of each slice. Markers on the anatomy or within the data can also provide the required position information. Yet another way is to estimate the scan plane displacements directly from the degree of speckle decorrelation between successive image frames.
If the ultrasound probe is swept over an area of a body (either by hand or by a probe mover) such that the inter-slice spacing is known, and if the slices are stored in memory, a three-dimensional data volume can be acquired. The data volume can be used to form a three-dimensional view of the area. of interest. In addition, the data can be reformatted to produce individual slices at arbitrary angles, thus allowing the user to get the exact view desired regardless of the anatomy under investigation.
The problem that arises is how to display the information in a way that makes it is easy for the observer to relate the two-dimensional slice to the three-dimensional anatomy. Commonly assigned Avila et al. U.S. Pat. No. 5,934,288, issued Aug. 10, 1999, discloses a system with multiple three-dimensional imaging modes that allow the user to view the data as a volume projection, or as individual slices at an arbitrary angle and location in the data volume. This allows the user to obtain any single slice desired and to scroll through the anatomy at that angle. Pat. No. 5,934,288 is hereby incorporated by reference. It is often desirable, however, to visualize how a slice through the data volume relates to non-parallel (e.g., orthogonal) slices through the same point in the data volume.
In an ultrasound imaging system which scans a human body and collects multiple images (e.g., B mode) in memory to form a data volume derived from a three-dimensional object volume, an image representing that data volume is displayed, accompanied by a plurality of images representing individual slices taken at different angles and intersecting at a point in the data volume. In a preferred embodiment, three images representing mutually orthogonal slices in the data volume are displayed. The display mode is hereinafter referred to as the xe2x80x9corthogonal cut planexe2x80x9d mode. However, those skilled in the pertinent art will readily appreciate that the cut planes need not be mutually orthogonal.
After acquiring a data volume and defining a region of interest, the user enters the orthogonal cut plane mode. This mode may be one of multiple three-dimensional imaging modes or it may be the sole three-dimensional imaging mode. If multiple modes exist, preferably the initial mode on entry to the three-dimensional imaging mode is the orthogonal cut plane mode. The initial mode as well as the sequence of progression from one mode to the next are set in software. Those skilled in the pertinent art will readily appreciate that the orthogonal cut plane mode need not be the initial mode.
The orthogonal cut plane mode provides the user with a projection of the data volume along with three orthogonal cut planes through the data volume. In addition, an orientation box is displayed to aid the user in visualizing the data volume orientation and the position of the slices in the data volume, and to identify which one of the four displays can be manipulated with the trackball (hereinafter referred to as the xe2x80x9cactivexe2x80x9d display). When the orthogonal cut plane mode is initially entered, the volume projection is active and the orientation box is therefore shown in green and the cut planes are each displayed in a different color (e.g., red, blue and yellow). Additionally, each cut plane display is framed in its corresponding color. When the user manipulates the trackball, the data volume projection is updated in real time. Pressing the xe2x80x9ccursorxe2x80x9d key changes the xe2x80x9cactivexe2x80x9d display to the next one in sequence and changes its frame to green. At the same time the wire frame for the volume projection changes to white. Manipulating the trackball when one of the cut planes is active allows the user to scroll through the data volume in the selected orientation.